TLUG Talk: Storage Technologies and Theory. RAID

So what is a RAID array? Being here you probably have enough interest in computers to have heard of RAID but unless you are slightly obsessed with hard drive technology you probably haven’t learned much about it. RAID has been, and to a large part still is the domain of higher-level servers.

RAID describes three main abilities that can be implemented either alone or in combination to best fit various scenarios. These features include «stripping», «mirroring» and «parity».

Stripping, known as RAID level 0 or RAID0 is the process of using two or more drives for simultaneous writing and reading. When a file is to be written to a stripped array the data is divided into chunks and written to the drives in the array at the same time. As a loose example you can take a 10MB file and write it to a RAID0 array with two drives in roughly the time it would normally take to write a 5MB file (twice the speed). The same 10MB file could be written to an array with five drives in roughly the time it would have taken a 2MB file to be written to a single drive (five times as fast). Calculating the actual speed benefits isn’t so cut and dry because of other overhead but you get a good idea.

Next up is «Mirroring» or RAID1. As its name implies, two drives are mirror images of one another. If one drive fails the data is safe thanks to the second identical drive. The down side is that 50% of the physical hard drive space is wasted.

Finally we get to «Parity», used in RAID3, 4, 5 and 6 but most popularly in RAID5. Remember in math class you asked «where will I ever use this in the real world?» Well my friends, Boolean algebra has allowed us a very efficient was to protect data. Lets use a RAID5 array for this example but first let me describe a RAID5 array.

In a RAID5 array you need a minimum of 3 disks. The more you add though the better performance you gain and the more efficiently you use your disk space. The trade off is you need an increasingly more powerful RAID controller and that translates to a higher cost. In a RAID5 array performance is increased by stripping data across the available drives (RAID0). In a RAID0 array though a single disk failure will destroy all the data because part of just about every file is on each disk. Parity is added in RAID5 to deal with this.